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HOME > PAST ISSUE > March-April 2011 > Article Detail

MARGINALIA

One Shocked Chemist

Molecular surprises are sometimes right in front of us, if only we’d do the math

Roald Hoffmann

So Why Is it so Hard to Make?

If something is thermodynamically stable, one should be able to make it. Yes, but….

2011-03MargHoffmannFE.jpgClick to Enlarge ImageWhat matters in chemistry is not thermodynamic stability, but kinetic persistence. Chemistry is the land of thermodynamically stable or (more interesting) unstable molecules that have high barriers to going to where they (or we) want to go. For example, nearly every molecule in our bodies—with the exception of H2O, CO2, phosphate and some other small ions—is thermodynamically unstable in the presence of oxygen. Were it not for the water in us, and the high barriers to oxidation, we should burn very nicely. Literally, not just with passion.

Another example: Of the four most stable (with respect to separation into atoms) diatomic molecules that are made of identical atoms—N2, C2, P2, O2—only two are available in a bottle at room temperature and pressure. Whereas other diatomics, much less stable to atomization, are there at ambient conditions—F2 and Cl2 for instance.

Additionally, benzene itself has a positive heat of formation from the elements, graphite and diatomic hydrogen. It shows no hint of decomposing to them, of course. The barriers to initiating that decomposition are stupendous.

Graphane is a two-dimensional raft of CHs. Organic chemists, masters of parlaying designed complexity in zero-dimensional molecules into other zero- and one-dimensional molecules, have trouble exercising control in two and three dimensions. There are emerging exceptions, for instance in the self-assembled complexity of metal-organic frameworks.

Graphane is slightly more stable than benzene, but there is no systematic, bond-by-bond route to graphane (nor was there to buckminsterfullerene). Elias and his colleagues, in their approach to perfect graphane, reacted a suspended graphene sheet with a plasma of hydrogen.

The reactions forming graphane from benzene in the fourth figure were a Gedankenexperiment, not a synthetic route. To be sure, they are all “allowed,” in the sense that Robert B. Woodward and I delineated some years ago. But entropy factors aside, they are nevertheless certain to have substantial barriers to proceeding—bonds have to be broken partially in the initial stages of the reaction, and that costs energy. So, a Diels-Alder cycloadditions of ethylene and butadiene to cyclohexene, a prototype-allowed reaction, still has an activation energy of 115 kilojoules per mole. The fifth figure shows two allowed fragmentations that are highly exothermic, yet have energy barriers of about 65 kilojoules per mole. Benzenes are very, very unlikely to form graphane in the way the fourth figure shows.

I am sure pure graphanes will be made, in some nonsporting yet reproducible way. I use the plural, because the three structures drawn above are stereoisomers (they differ in the three-dimensional arrangements of their atoms in space) and although not very different in energy, face gigantic barriers to conversion amongst themselves. And, once made, graphanes will not decompose spontaneously to benzene. Even as kinetics is what matters, thermodynamics rules.

Acknowledgments

I am very grateful to Sason Shaik for correcting some of my thinking, to Jerome Berson for a comment and to Xiao-Dong Wen for his research and illustrations.

©Roald Hoffman

Bibliography

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